Record Polymorphism

SML# supports record polymorphism, which makes SML's field selection operators #<label> and record patterns fully polymorphic, as seen in the following example:

 #  fun f x = #name x;
 val f = fn : ['a, 'b#{name:'a}. 'b -> 'a]
 #  f {name = "Joe", age = 21};
 val it = "Joe" : string

where ['a, 'b#{name:'a}.'b -> 'a] indicates that f is a polymorphic function operating on any record containing a name field of some type 'a and returns that value of type 'a.

In addition to those simple filed selection, SML# also allow polymorphic functional filed update operation by the syntax

 exp # {label1 = exp1, ..., labeln = expn}

that creates a new record from the given record expression exp by changing its labeli value to expi. The folloing is a useful ideom frequently appearing in a program manipulating records.

#  fun incX (x as {X,...}) = x # {X = X + 1};
val incX= fn : ['a#{X:int}.'a -> 'a}
#  incX {X = 1, Y = 2};
val it = {X = 2, Y = 2} : {X:int, Y:int}

Moreover, the SML# compiler always compiles thoese polymorphic record operations into efficient load instructions with an integer offset. The following is an example compilation.

# fun makeGreeting {name= {first = "David", ...},...} = "Hi, Dave"
>   | makeGreeting {name = {last,...}, ...} =  "Dear Mr./Ms." ^ last;

 Record Compiled to:
 val makeGreeting =
   ['a#{name:'b},'b#{first:string, last:string}.
     fn {$175, $174, $173, $167} =>
       bind
         $170 = $167[$175]
       in
          bind
            $171 = $170[$174]
          in
             switch
               $171
             of "David" => "Hi, Dave"
              | _ =>
                  bind
                    last = $170[$173]
                  in
                    "Dear Mr./Ms." ^ last
                  end
          end
       end
   ]

makeGreeting function is compiled to a function that takes a record ($167) together with three integer offsets ($173,$174, and $175) for name, first, and last fields, and accesses these feilds directly with the offet values.